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Scope: The recent publication by Yin (2013; Annals of Botany 112: 465-475) referred to in the title above provides an excellent review of modelling approaches to predict the impact of elevated CO2 on crop productivity, as well as on the controversy regarding whether yield responses observed in free-air CO2 enrichment (FACE) experiments are indeed lower than those from chamber-based experiments. However, the wheat experiments in the example of fig. 1 in Yin's paper had a flaw as the control plots lacked blowers that were in the FACE plots, which warmed the FACE plots at night and hastened plant development. This Viewpoint seeks to highlight this fact, and to comment on the relative merits of FACE and enclosure experiments.

The positive impact of elevated atmospheric CO(2) concentration on crop biomass production suggests more carbon inputs to soil. Further study on the effect of elevated CO(2) on soil carbon and nitrogen dynamics is key to understanding the potential for long-term carbon storage in soil. Soil samples (0- to 5-, 5- to 10-, and 10- to 20-cm depths) were collected after 2 yr of grain sorghum [Sorghum bicolor (L.) Moench.] production under two atmospheric CO(2) levels: (370 [ambient] and 550 muL L(-1) [free-air CO(2) enrichment; FACE]) and two water treatments (ample water and limited water) on a Trix clay loam (fine, loamy, mixed [calcareous], hyperthermic Typic Torrifluvents) at Maricopa, AZ. In addition to assessing treatment effects on soil organic C and total N, potential C and N mineralization and C turnover were determined in a 60-d laboratory incubation study. After 2 yr of FACE, soil C and N were significantly increased at all soil depths. Water regime had no effect on these measures. Increased total N in the soil was associated with reduced N mineralization under FACE. Results indicated that potential C turnover was reduced under water deficit conditions at the top soil depth. Carbon turnover was not affected under FACE, implying that the observed increase in soil C with elevated CO(2) may be stable relative to ambient CO(2) conditions. Results suggest that, over the short-term, a small increase in soil C storage could occur under elevated atmospheric CO(2) conditions in sorghum production systems with differing water regimes.

The developmental pattern of C4 expression has been well characterized in maize and other C4 plants. However, few reports have explored the possibility that the development of this pathway may be sensitive to changes in atmospheric CO2 concentrations. Therefore, both the structural and biochemical development of leaf tissue in the fifth leaf of Sorghum bicolor plants grown at elevated CO2 have been characterized. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) activities accumulate rapidly as the leaf tissue differentiates and emerges from the surrounding whorl. Rubisco was not expressed in a cell-specific manner in the youngest tissue at the base of the leaf, but did accumulate before PEPC was detected. This suggests that the youngest leaf tissue utilizes a C3-like pathway for carbon fixation. However, this tissue was in a region of the leaf receiving very low light and so significant rates of photosynthesis were not likely. Older leaf tissue that had emerged from the surrounding whorl into full sunlight showed the normal C4 syndrome. Elevated CO2 had no effect on the cell-specific localization of Rubisco or PEPC at any stage of leaf development, and the relative ratios of Rubisco to PEPC remained constant during leaf development. However, in the oldest tissue at the tip of the leaf, the total activities of Rubisco and PEPC were decreased under elevated CO2 implying that C4 photosynthetic tissue may acclimate to growth under elevated CO2.

Every two months of 1992, as well as on three occasions in 1994-1995, we obtained leaf samples together with samples of surrounding air from eight well-watered and fertilized sour orange (Citrus aurantium L.) trees that were growing out-of-doors at Phoenix, Arizona, USA. These trees had been planted in the ground as small seedlings in July of 1987 and enclosed in pairs by four clear-plastic-wall open-top chambers of which two have been continuously maintained since November of that year at a CO2 concentration of 400 micromol mol(-1) and two have been maintained at 700 micromol mol(-1). In September 2000, we also extracted north-south and east-west oriented wood cores that passed through the center of each tree's trunk at a height of 45 cm above the ground. Stable-carbon isotope ratios (13C/12C) derived from these leaf, wood and air samples were used to evaluate each tree's intrinsic water-use efficiency (iWUE). The grand-average result was an 80% increase in this important plant parameter in response to the 300 micromol mol(-1) increase in atmospheric CO2 concentration employed in the study. This increase in sour orange tree iWUE is identical to the long-term CO2-induced increase in the trees' production of wood and fruit biomass, which suggests there could be little to no change in total water-use per unit land area for this species as the air's CO2 content continues to rise. It is also identical to the increase in the mean iWUE reported for 23 groups of naturally occurring trees scattered across western North America that was caused by the historical rise in the air's CO2 content that occurred between 1800 and 1985.

Over the past decade, free-air CO2 enrichment (FACE) experiments have been conducted on several agricultural crops: wheat(Triticum aestivum L.), perennial ryegrass (Lolium perenne), and rice(Oryza sativa L.) which are C3 grasses; sorghum (Sorghum bicolor (L.) Möench), a C4 grass; white clover (Trifolium repens), a C3 legume; potato (Solanum tuberosum L.), a C3 forb with tuber storage; and cotton (Gossypium hirsutum L.) and grape (Vitis vinifera L.) which are C3 woody perennials. Using reports from these experiments, the relative responses of these crops was discussed with regard to photosynthesis, stomatal conductance, canopy temperature, water use, water potential, leaf area index, shoot and root biomass accumulation, agricultural yield, radiation use efficiency, specific leaf area, tissue nitrogen concentration, nitrogen yield, carbohydrate concentration, phenology, soil microbiology, soil respiration, trace gas emissions, and soil carbon sequestration. Generally, the magnitude of these responses varied with the functional type of plant and with the soil nitrogen and water status. As expected, the elevated CO2 increased photosynthesis and biomass production and yield substantially in C3 species, but little in C4, and it decreased stomatal conductance and transpiration in both C3 and C4 species and greatly improved water-use efficiency in all the crops. Growth stimulations were as large or larger under water-stress compared to well-watered conditions. Growth stimulations of non-legumes were reduced at low soil nitrogen, whereas elevated CO2 strongly stimulated the growth of the clover legume both at ample and under low N conditions. Roots were generally stimulated more than shoots. Woody perennials had larger growth responses to elevated CO2, while at the same time, their reductions in stomatal conductance were smaller. Tissue nitrogen concentrations went down while carbohydrate and some other carbon-based compounds went up due to elevated CO2, with leaves and foliage affected more than other organs. Phenology was accelerated slightly in most but not all species. Elevated CO2 affected some soil microbes greatly but not others, yet overall activity appears to be stimulated. Detection of statistically significant changes in soil organic carbon in any one study was impossible, yet combining results from several sites and years, it appears that elevated CO2 did increase sequestration of soil carbon. Whenever possible, comparisons were made between the FACE results and those from prior chamber-based experiments reviewed in the literature. Over all the data and parameters considered in this review, there are only two parameters for which the FACE- and chamber-based data appear to be inconsistent. One is that elevated CO2 from FACE appears to reduce stomatal conductance about one and a half times more than observed in prior chamber experiments. Similarly, elevated CO2 appears to have stimulated root growth relatively more than shoot growth under FACE conditions compared to chamber conditions. Nevertheless, for the most part, the FACE- and chamber-based results have been consistent, which gives confidence that conclusions drawn from both types of data are accurate. However, the more realistic FACE environment and the larger plot size have enabled more extensive robust multidisciplinary data sets to be obtained under conditions representative of open fields in the future high-CO2 world.

The article presents a study which aims to investigate the cumulative effect of 2 years of free-air carbon dioxide enrichment (FACE) and soil water stress in a grain sorghum production system on oil N and C internalization and C turnover. In this study, a FACE application system was employed in order to achieve carbon dioxide enrichment in the field under two soil water regimes for two growing seasons. For the soil variables at the depth intervals evaluated, no significant treatment interactions were noted.

Summary • The likely consequences of future high levels of atmospheric CO2 concentration on wheat (Triticum aestivum L.) grain nutritional and baking quality were determined. • Two free-air CO2 enrichment (FACE; 550 mmol mol-1 ) experiments were conducted at ample (Wet) and limiting (Dry) levels of irrigation, and a further two experiments at ample (High-N) and limiting (Low-N) nitrogen concentrations. Harvested grain samples were subjected to a battery of nutritional and bread-making quality tests. • The Dry treatment improved grain quality slightly (protein +2%; bread loaf volume +3%). By contrast, Low-N decreased quality drastically (protein -36%; loaf volume -26%). At ample water and N, FACE decreased quality slightly (protein -5%; loaf volume -2%) in the irrigation experiments and there was no change in the nitrogen experiments. At Low-N, FACE tended to make the deleterious effects of Low-N worse (protein -33% and -39%, at ambient CO2 and FACE, respectively; loaf volume -22% and -29% at ambient CO2 and FACE, respectively). • The data suggest that future elevated CO2 concentrations will exacerbate the deleterious effects of low soil nitrogen on grain quality, but with ample nitrogen fertilizer, the effects will be minor. [ABSTRACT FROM AUTHOR]

AbstractSpring wheat [Triticum aestivum (L). cv. Yecora Rojo] was grown from December 1992 to May 1993 under two atmospheric CO2 concentrations, 550 μmol mol–1 for high-CO2 plots, and 370 μmol mol–1 for control plots, using a Free-Air CO2 Enrichment (FACE) apparatus. In addition to the two levels of atmospheric CO2, there were ample and limiting levels of water supply through a subsurface trip irrigation system in a strip, split-plot design. In order to examine the temporal and spatial root distribution, root cores were extracted at six growth stages during the season at in-row and inter-row positions using a soil core device (86 mm ID, 1.0 m length). Such information would help determine whether and to what extent root morphology is changed by alteration of two important factors, atmospheric CO2 and soil water, in this agricultural ecosystem. Wheat root growth increased under elevated CO2 conditions during all observed developmental stages. A maximum of 37% increase in total root dry mass in the FACE vs. Control plots was observed during the period of stem elongation. Greater root growth rates were calculated due to CO2 enhancement until anthesis. During the early vegetative growth, root dry mass of the inter-row space was significantly higher for FACE than for Control treatments suggesting that elevated CO2 promoted the production of first-order lateral roots per main axis. Then, during the reproductive period of growth, more branching of lateral roots in the FACE treatment occurred due to water stress. Significant higher root dry mass was measured in the inter-row space of the FACE plots where soil water supply was limiting. These sequential responses in root growth and morphology to elevated CO2 and reduced soil water supports the hypothesis that plants grown in a high-CO2 environment may better compensate soil-water-stress conditions. [ABSTRACT FROM AUTHOR]

Sour orange trees have been grown from the seedling stage out-of-doors at Phoenix, Arizona, USA, in open- top enclosures with clear plastic walls for 3·5 years. For the last 3 years of this period, half of the trees have been continuously exposed to air enriched with CO2 to 300 μmol mol1 above the ambient concentration. At 2-month intervals over the last 12 months, we have determined the fine-root blomass in the top O·4m of the soil profile beneath the trees. Results from both treatments define a single relationship between fine- root biomass and trunk cross-sectional area. The data also show the CO2-enriched trees to have approximately 2·3 times more fine-root biomass in this soil layer than the trees grown in ambient air. [ABSTRACT FROM AUTHOR]

Soil water deficits are likely to influence the response of crop growth and yield to changes in atmospheric CO2 concentrations (Ca), but the extent of this influence is uncertain. To study the interaction of water deficits and Ca on crop growth, the ecosystem simulation model ecosys was tested with data for diumal gas exchange and seasonal wheat growth measured during 1993 under high and low irrigation at Ca = 370 and 550 µmol mol-1 in the Free Air CO2 Enrichment (FACE) experiment near Phoenix, AZ. The model, supported by the data from canopy gas exchange enclosures, indicated that under high irrigation canopy conductance (gc) at Ca = 550 µmol mol-1 was reduced to about 0.75 that at Ca = 370 µmol mol-1, but that under low irrigation, gc was reduced less. Consequently when C, was increased from 370 to 550 µmol mol-1 canopy transpiration was reduced less, and net CO2 fixation was increased more, under low irrigation than under high irrigation. The simulated effects of Ca and irrigation on diurnal gas exchange were also apparent on seasonal water use and grain yield. Simulated vs. measured seasonal water use by wheat under high irrigation was reduced by 6% vs. 4% at C, = 550 vs. 370 µmol mol-1 but that under low irrigation was increased by 3% vs. 5%. Simulated vs. measured grain yield of wheat under high irrigation was increased by 16% vs. 8%, but that under low irrigation was increased by 38% vs. 21%. In ecosys, the interaction between Ca and irrigation on diumal gas exchange, and hence on seasonal crop growth and water use, was attributed to a convergence of simulated gc towards common values under both Ca as canopy turgor declined. This convergence caused transpiration to decrease comparatively less, but CO2 fixation to increase comparatively more, under high vs. low Ca. Convergence of gc was in turn attributed to improved turgor maintenance under elevated Ca caused by greater storage C concentrations in the leaves, and by greater rooting density in the soil. [ABSTRACT FROM AUTHOR]

The effects of elevated carbon dioxide (CO2) concentration on plant water use are best evaluated on plants grown under field water use are best evaluated on plants grown under field conditions and with measurement techniques that do not disturb the natural function of the plant. Heat balance sap flow gauges were used on individual main stems of wheat (Triticum aestivum L. cv Yecora rajo) grown under normal ambient conditions (control) and in a free-air CO2 enrichment (FACE) system in Arizona with either high (control + high H2O=CW; FACE+high H2O=FW) or low (control+low H2O=CD; FACE+low H2O=FD) irrigation regimens. Over a 30 d period (stem elongation to anthesis), combinations of treatments were monitored with 10-40 guages per treatment. The effects of increased CO2 on tiller water used were inconsistent in both the diurnal patterns of sap flow and the statistical analyses of daily sap flow (Ftot). Initial results suggested that the reduction in Ftot from CO2 enrichment were small (0-10%) in relation to the H2O treatment effect (20-30%). For a 3d period, Ftot of FW was 19-26% less than that of CW (P=0.10). Examination of the different sources of variation in the study revealed that the location of gauges within the experimental plots influenced the variance of the sap flow measurements. This variation was probably related to positional variation in subsurface drip lines used to irrigate plots. A sampling design was proposed for use of sap flow gauges in FACE systems with subsurface irrigation that takes into account the main treatment effect of CO2 enrichment and the other sources of variation identified in this study. Despite the small and often statistically non-significant differences in Ftot between the CW and FW treatments, cumulative water use of the FW treatment at the end of the first three test periods ranged from 7 to 23% lower than that of the CW treatment. Differences in sap flow between FW and CW compared well with treatment differences in evapotranspiration. The results of the study, based on the first reported sap flow measurements of wheat, suggest that irrigation requirements for wheat production in the present climatic regimen of the south-western US, may be predicted to decrease slightly because of increasing atmospheric CO2. [ABSTRACT FROM AUTHOR]

A spring wheat crop was grown at ambient and elevated (550 μmol mol-1) CO2 concentrations under free-air CO2 enrichment (FACE) in the field. Four experimental blocks, each comprising 21-m-diameter FACE and control experimental areas, were used. CO2 elevation was maintained day and night from crop emergence to final grain harvest. This experiment provided a unique opportunity to examine the hypothesis that CO2 elevation in the field would lead to acclimatory changes within the photosynthetic apparatus under open field conditions and to assess whether acclimation was affected by crop developmental stage, leaf ontogeny and leaf age. Change in the photosynthetic apparatus was assessed by measuring changes in the composition of total leaf and thylakoid polypeptides separated by SDS-PAGE. For leaves at completion of emergence of the blade, growth at the elevated CO2 concentration had no apparent effect on the amount of any of the major proteins of the photosynthetic apparatus regardless of the leaf examined. Leaf 5 on the main stem was in full sunlight at emergence, but then became shaded progressively as 3-4 further leaves formed above with continued development of the crop. By 35 d following completion of blade emergence, leaf 5 was in shade. At this point, the chlorophyll a/b ratio had declined by 26% both in plants grown at the control CO2 concentration and in those grown at the elevated CO2 concentration, which is indicative of shade acclimation. The ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content declined by 45% in the control leaves, but by 60% in the leaves grown at the elevated CO2 concentration. The light- harvesting complex of photosystem II (LHCII) and the chlorophyll content showed no decrease and no difference between treatments, indicating that the decrease in Rubisco was not an effect of earlier senescence in the leaves at the elevated CO2 concentration. Following completion of the emergence of the flag-leaf blade, the elevated-CO2 treatment inhibited the further accumulation of Rubisco which was apparent in control leaves over the subsequent 14d. From this point onwards, the flag leaves from both treatments showed a loss of Rubisco, which was far more pronounced in the elevated-CO2 treatment, so that by 36 d the Rubisco content of these leaves was just 70% of that of the controls and by 52d it was only 20%. At 36d, there was no decline in chlorophyll, LHC!I or the chloroplast ATPase coupling factor (CFI) in the elevated CO2 concentration treatment relative to the control. By 52 d, all of these proteins showed a significant decline relative to the control. This indicates that the decreased concentration of Rubisco at this final stage probably reflected earlier senescence in the elevated-CO2 treatment, but that this was preceded by a CO2-concentration-dependent decline in Rubisco. [ABSTRACT FROM AUTHOR]

Pinus eldarica L. trees, rooted in the natural soil of an agricultural field at Phoenix, Arizona, were grown from the seedling stage in clear-plastic-wall open-top enclosures maintained at four different atmospheric CO2 concentrations for 15 months. Light response functions were determined for one tree from each treatment by means of whole-tree net CO2 exchange measurements at the end of this period, after which rates of carbon assimilation of an ambient-treatment tree were measured across a range of atmospheric CO2 concentrations. The first of these data sets incorporates the consequences of both the CO2- induced enhancement of net photosynthesis per unit needle area and the CO2-induced enhancement of needle area itself (due primarily to the production of more needles), whereas the second data set reflects only the first of these effects. Hence the division of the normalized results of the first data set by the normalized results of the second set yields a representation of the increase in whole-tree net photosynthesis due to enhanced needle production caused by atmospheric CO2 enrichment. In the solitary trees we studied, the relative contribution of this effect increased rapidly with the CO2 concentration of the air to increase whole-tree net photosynthesis by nearly 50% at a CO2 concentration approximately 300 μmol mol-1 above ambient. [ABSTRACT FROM AUTHOR]

Eight sour orange trees have been grown from seedling stage in the field at Phoenix, Arizona. U. S. A., in tour identicaliy-vented, open-top, clear-plastic-wall chambers for close to 2.5 years. Halt of the chambers have been maintained at ambient atmospheric CO2 concentrations over this period, while half of them have been maintained at 300 ppm (300 μmol CO2 per mol air) above ambient. Initially, the trees in each treatment were essentially identical, but in less than 2 years, the trunks of the CO2-enriched trees had become twice as large as their ambient-treatment counterparts. After 2 full years of growth, the enriched trees had 79% more leaves, 56% more primary branches with 172% more volume, 70% more secondary branches with 190% more volume, and 240% more tertiary branches with 855% more volume. In addition, the CO2-enriched trees also had fourth-, fifth- and sixth-order branches, while the ambient-treatment trees had no branches above third order. Total trunk plus branch volume of the CO2-enriched trees was 2.79 times that of the ambient-treatment trees after 2 full years of growth. [ABSTRACT FROM AUTHOR]

The Maricopa cotton and wheat FACE (free-air CO2 enrichment) experiments offer propitious opportunity to quantify carbon turnover. The commercial CO2 (delta13C ~~ -37%) usedto elevate CO2 concentration in field plots provided a strongly 13C-depleted tracer. Soil CO2 and delta13C of soil organic carbon (SOC) in CO2-enrichedand control plots were measured between the final cotton FACE project (October 1991) and the end of the second wheat experiment (June 1994). The initial 13C depletion in SOC of cotton FACE plots (measured by the difference in delta13C between FACE and Control plots) persisted at the same level (1.9%) 1.5 years after the experiment ended. A similar depletion was observed in Soil CO2 evolved in the same plots, indicating ongoing decomposition of the new SOC. The SOC delta13C of wheat plots before and after two growing seasons showed increasing 13C-depletion in FACE relative to Control. Isotopic mass balance was consistent with 5--6% new carbon input from the two wheat crops. This is lower than the 12--13% calculated for FACE cotton and perhaps a consequence of thelarger root system of cotton or the 3-year duration of the cotton experiments versus 2 years for the wheat. [ABSTRACT FROM AUTHOR]